Project with PIC and Loadcell

[falcon _bd]

I am trying to build a weight scale with 16F877A and a loadcell (model LHD00-4) 750kg.
I need to know how I can connect the load cell with pic. Here is the load cell specification.
What types of  output of this load cell , resistive or directly mV ?




http://www.hand-free.cn/1-5loadcell.htm#4

[Wizpic]

Looking at the data sheet , I would say it gives a voltage out, I've looked on the web and I can see that it says Supply Voltage V(DC) 6~15V
so I would say it gives a voltage out, If you have got one the only thing that I could say is try it connect it up and  first and put a known wieght say 10KG on it and see what the ouput voltage it gives then double the wieght then check again and do the same again, By doing it this way it should make it easier for you to work out your calculations, If the voltage is greater then 5V then just use an op-amp or resistor network to scale the max volatge down to 5V,

How may wires come out of it, I've seen these fitted to some machines and if I remeber there is 5 or 6 wires ? am I correct in thinking that there is a bolt or someting that passes throught the middle which tells how much wait there is

[falcon _bd]

I already checked it. When I applied supply voltage 15v, weight 10kg the out put is 0.1V. when weight 70kg out put is 0.2V. And I cant increase the output if I apply double weight.

There are 5 wire . I/p + , I/p - , O/p +, O/p -, earth

What can I do now 

[madhan7710]

Hi,

You need to amplify the output to directly connect to the ADC of the PIC. You can use INA101 or INA125. you can use any voltage multiplier circuit or else use a separate high resolution ADC (24 bit) with internal PGA (Programmable gain amplifier) as this will give accurate result. If you can work on 8051 core then u can choose TI's msc1210 which has 24 bit internal ADC with PGA with 8051 micro. You can directly connect the loadcell to the controller.

[MGLSOFT]

The resolution of a load cell is defined by the manufacturer.
Normally this is a voltage proportional to the voltage applied to their power cables.
In your case it is 2 mV / V, ie the maximum output voltage of 15 volts applied to the input is 30 mV.
For a load of 70 Kg and 750 Kg of a cell fed with 15 Volts, the output is 2.8 mV.
750/70 * 30 = 2.8

 

[chandra2sekhar2000]

hai,
u can use analog devices ic AD7799 .IT HAS INTERNAL AMPLIFIER AND ADC .USE THE SPI OUTPUT OF ADC TO MICRO .U CAN FIND MORE INFORMATION IN LINK BELOW

http://www.analog.com/library/analogDialogue/archives/39-12/weigh_scale.pdf

[charudatt]

There are some other designs as well , which are tropicalised (I mean Indian Design - Cheap !) using a 1$ chip. Take a look at this post from Proton Basic Forum where, P.N. Shaji has made some design using CS5460 IC and this design is very famous locally - INDIAN. The PCB used to cost arround Rs.700/- in kit form. It used AT89C52.

http://www.picbasic.org/forum/showthread.php?t=7252&highlight=load+cell

I wanted to convert it for the PIC , but did not get a chance.

Posting this information, just in case if, some one was interested.

regards
 

[vinay.gvc]

I have made many commercial projects for weighing machine. The load cell outputs are define as 5mV/V . So if a load cell is meant for 10KG and you provide 5V DC, at 10KG output will be 25mV. You need a 20 bit  ADC to read in grams.

The current controller is NAC7802 and is available in Indian local markets for Rs 30/- each. This is a 24 bit ADC and can be read using I2C bus thru almost all micro controllers. I have used freescale's 9S08, Renesas R8C/1B and not using STM8S003.

Porting it to PIC will not be complex task. Read the data sheet and it will be clear to you.

Need any specific help, let me know.

Will be glad to help.

[cerebronico]

can't find reference to NAC7802, may you post your code for zeroing, span, etc?

[Vineyards]

It looks easy on paper but noise can be a big problem in a load cell Project. Especially so, if we are talking about using a 20bit ADC.
I think these are needed:
A differential input or better still a ratiometric input.
with temperature compensation
perhaps a non-linear gain stage to handle the large signal range.